Purpose: Participation in cancer support groups can provide a sense of community and may better prepare patients for interactions with their health care team. Online interactions may overcome some barriers to in-person support group participation. #BCSM (breast cancer social media), the first cancer support community established on Twitter, was founded in 2011 by two breast cancer survivors. The aims of this study are to describe the growth and changes in this community and to discuss future directions and lessons that may apply to other online support communities. Methods: Symplur Signals was used to obtain all #BCSM Twitter data from January 1, 2011, to January 1, 2020 (00:00:00 Coordinated Universal Time for both). Hashtag use by selected stakeholder groups, user locations, weekly tweet chat activity, and topics were determined. Results: From year 1 (2011) to year 9 (2019), tweets using the #BCSM hashtag increased by 424%. Tweets by patient advocates increased by 226%, with a peak i